=htc #17683 drop already consumed input in continue

akka-http-core/src/main/scala/akka/http/impl/engine/parsing/HttpMessageParser.scala

@@ -252,7 +252,9 @@ private[http] abstract class HttpMessageParser[Output >: MessageOutput <: Parser
   def continue(input: ByteString, offset: Int)(next: (ByteString, Int) ⇒ StateResult): StateResult = {
     state =
       math.signum(offset - input.length) match {
-        case -1 ⇒ more ⇒ next(input ++ more, offset)
+        case -1 ⇒
+          val remaining = input.drop(offset)
+          (more ⇒ next(remaining ++ more, 0))
         case 0 ⇒ next(_, 0)
         case 1 ⇒ throw new IllegalStateException
       }